“Atopic reagin” is the name given by Coca to the specific, reacting body discovered by Prausnitz and Küstner in the blood serum of atopic individuals. These reagins have been studied by a number of investigators (1, 2, 3, 4) and their existence thoroughly proved. Briefly, they can be demonstrated by injecting the blood serum from a person suffering from asthma (or an allied condition) into the skin of a normal individual. At the site of this injection, atopens or substances which gave a positive skin reaction in the skin of the donor of the serum will also give a positive reaction in the normal skin of the recipient but will give a negative reaction at any other site not previously injected. Coca and Grove (3) found that, in a mixture of appropriate atopen and reagin containing serum, the reagin was neutralized. That is they were unable to sensitize a normal skin with such a mixture.